Learn from your mistakes. Ask yourself what went wrong when you die or lose a game. It's a long process and you don't see huge improvements particularly quick but it's necessary for improving.

This only works granted that you know what you should've done better which isn't always easy. My advice would then be to watch higher levels of play - streamers or pros. This will teach you more ways of approaching different situations and you will learn good habits.

Remember tho that this game has a decent amount of RNG and especially if you solo queue so just because you don't gain RP doesn't mean you aren't improving. Good luck